Power boilers have long been used by industries and utilities to produce steam for power production and process requirements. These boilers come in many types and sizes but embodiments of the present invention are an improvement on bed-fired water tube boilers ranging in steam production from 25 tons per hour to 300 tons per hour or more. The fuel may consist of bark, sawdust, wood chips, biomass trimmings, wood or other biomass pellets, urban waste, tire derived fuel (TDF), crushed coal, pet coke, sludge, fiber line rejects, straw based fuels, or other solid fuel, or a combination of fuels, and may have moisture content as high as 60%. These boilers are typically constructed of heavy wall steel tubes welded side by side into wall panels that form the front, rear and side walls of the boiler. The lower portion of this box forms the combustion chamber of the boiler and is sometimes called the furnace. The tubes are typically 2½″ to 3″ in outside diameter with a wall thickness from 0.18″ to 0.25″ and spaced apart 3″ to 4″ center to center. The gaps between the tubes are filled with steel strips about ¼″ thick by the width of the gap. The entire panel is seal-welded air tight. The lower ends of the wall tubes are welded into larger diameter horizontal header pipes that feed water to the walls. The tops of the wall tubes are also connected to larger diameter horizontal collector pipes that carry the water away from the walls to the steam drum, located at the top of the boiler. The front wall tubes are typically bent over to form the roof of the boiler and those tubes can terminate in a collector pipe or directly to the steam drum. Similarly the rear wall tubes are typically bent to create a “bullnose” or “nose arch” to direct combustion gasses across the convective section of the boiler and then terminate in a water drum, steam drum, or collector pipe at the top of the boiler. The top of the bullnose is usually at the elevation of the water drum. The convective section generally consists of a set of superheaters, located at the top of the boiler, that are heated predominantly through convection. Downcomer pipes connect the steam drum or water drum at the top of the boiler to the header pipes at the bottom of the tube walls and feed water from the drum to the walls. The bottom of the boiler can be a travelling or vibrating grate, tilting grate, sloping grate, step grate, fluidized bed, or a stepped floor as described in U.S. patent application Ser. No. 12/557,085. Fuel enters the boiler through a chute or chutes penetrating one or more walls of the boiler and may be broadcast into the boiler by a fuel distributor, for example, as described in U.S. patent application Ser. No. 12/406,035. The fuel falls to the floor or grate where it is mixed with air and burns. The heat released by the burning fuel is absorbed by the wall tubes and heats the water in the walls, where the water expands thermally and starts to boil. The heated and boiling water is less dense than the water in the downcomer pipes; therefore a natural circulation is created with hotter water rising in the tube walls and cooler water descending in the downcomer pipes. The natural circulation is an inherent safety feature of these boilers as the circulation rate increases as more fuel is burned and more heat released in the combustion chamber.
As the water circulates from the steam drum, down through the downcomers, up through the walls, and back to the steam drum, some of the water may boil but most of the boiling occurs in the steam generating bank, sometimes called the boiler bank. In older two drum boilers, the generating bank is a set of tubes connecting the bottom of the steam drum to the top of a water drum, sometimes called a mud drum, located up to thirty feet or so directly below the steam drum. There are generally hundreds of tubes connecting the two drums. The generating bank is arranged so that hot gasses from the furnace flow across the tubes and heat the water circulating inside. About half of the tubes in the generating bank of a two drum boiler are up flow tubes and the remainders are down flow tubes. The gas cools as it passes through the generating bank, therefore the first tubes the gas contacts (the front tubes as the gas flow through a boiler is generally front to back) are hotter and more boiling occurs in those tubes. The boiling water is less dense therefore the water circulates from the steam drum down through the rear tubes to the water drum then up through the front tubes back to the steam drum. The steam drum is generally about half full of water with saturated steam being released at the surface. The steam goes through a set of moisture separators and then to the superheaters. In newer single drum boilers there is no water drum, instead, the generating bank is fed by external (non-heated) downcomers from the steam drum and the water circulates down the downcomers and back up through all of the generating bank tubes to the steam drum. Single drum boilers are less expensive to build because the drums, especially with hundreds of tube penetrations, are the most expensive components. They also have other advantages including more flexible arrangements for locating the steam drum.
Some boilers also have sets of tubes located just at the furnace exit and arranged to cross the boiler at the top of the combustion chamber. These are called screen tubes or screens, and are often arrayed as platens in which several tubes are in close parallel arrangement, one on top of another, extending from the front or rear wall of the boiler through the opposite wall. These platens are generally separated 12″-15″ apart side to side and slope upward slightly to the other side of the boiler, or they may bend part way across the boiler and rise up vertically through the roof. The screen tubes are fed by external (non-heated) downcomers from the steam drum or water drum at their lower end and relieved back to the steam drum at their upper end. Water circulates from the steam drum or water drum through the screens and back up to the steam drum. The screens are located where the gasses are very hot and absorb heat predominantly by radiation.
After the steam leaves the steam drum it goes to the superheaters. These are sets of tubes typically located at the top of the boiler, above the screen tubes and in front of the generating bank. The superheaters increase the temperature of the steam from the saturation temperature in the steam drum to the final temperature desired for the process or power plant. The superheater tubes are typically arranged as vertical platens with up to a dozen tubes or more in close parallel arrangement front to back in each platen. There are many platens located across the width of the boiler with a spacing of 6″-15″ between platens. There are frequently three or more superheater sections with external (unheated) connecting pipes and/or desuperheaters between the sections. Desuperheaters or attemporators control the final steam temperature by spraying water into the steam, or other means. The superheater tubes start at the top of the boiler and drop vertically to just above the bullnose then run up and down a number of times before exiting back through the roof. The steam passes through the superheaters just once therefore the superheaters are not part of the boiler circulation circuits.
After the combustion gasses exit the generating bank they typically flow through an economizer or an air heater. Economizers are tube bundles either in cross flow or parallel flow to the gas stream through which the feedwater passes once and is heated and then goes to the steam drum. The feedwater flow is controlled to maintain the water level in the steam drum. Feedwater makes up for the steam that is produced and exits the boiler. Upon entry into the drum, feedwater is baffled and mixes with some of the water already within the steam drum to flow to the downcomer pipes or downcomer tubes. This feedwater mixed zone is colder and has higher density, which provides the driving head for the natural circulation in the boiler. The economizer may be located immediately after the generating bank (relative to the gas flow) integral with the boiler, or it may be located downstream from a tubular air heater or a dust collector.
Some of these boilers are supported from underneath (ground supported) but most, especially larger boilers, are hung from the top and expand downward as they heat up. A “hung” boiler typically may require a very strong and expensive structure to support the boiler. One of the biggest problems with current boiler design is the cost of erection. Smaller boilers are often supplied as a single unit or “package” boiler but larger boilers typically may be erected in the field. This frequently takes much longer and may be much more expensive than anticipated, driving up the actual cost of the boiler. To partially address this problem, some boilers have been “modularized” to speed up the construction and reduce the risk associated with assembling the boiler in the field. Embodiments of present invention incorporate some specific modular features to minimize the time and cost to erect the boiler.
Boilers as described above have been in use for many years and the technology is very mature, but they are very expensive and have significant operational limitations. Grate fired boilers and fluidized bed boilers are limited in the temperatures they can tolerate in the lower furnace otherwise they will over heat the grate or sand bed. They also do a poor job of mixing the combustion air and pyrolysis gasses above the bed because the air flow arrangement is dictated by the requirements to cool the grate or fluidize the sand bend. This leaves little setup flexibility to improve combustion in the boiler. Mechanical grates suffer from poor reliability and fluidized bed boilers suffer from excessive sand erosion and sand agglomeration. These deficiencies are addressed with the introduction of stepped floor and fuel drying chute technologies as described in U.S. patent application Ser. Nos. 12/557,085 and 12/471,081 respectively, and provisional application 61/522,939. Aspects of those technologies are incorporated into embodiments of the present invention to improve the combustion of difficult to burn fuels.